The present disclosure relates to a recording and reproducing apparatus for recording data to and reproducing data from a hologram recording medium and, in particular, to a defect determination method for determining the presence or absence of a defect occurring in an optical path.
In a hologram recording and reproducing method, in particular, the one using an optical storage, a spatial light modulator (SLM) such as a liquid-crystal panel or a digital micro-mirror device (DMD) is employed as a light intensity modulator to perform intensity modulation on a signal light ray to result in a pattern of bit 1 (high light intensity) and bit 0 (low light intensity). The signal light modulated in accordance with recording data is directed to a hologram recording medium together with light ray called reference light separately directed thereto. In this way, an interference pattern between the signal light and the reference light is recorded on the hologram recording medium.
During data reproducing operation, diffracted light responsive to the interference pattern is obtained by directing only the reference light to the hologram recording medium. An image responsive to the diffracted light is focused on an image sensor such as a charge-coupled device (CCD) or a complementary oxide semiconductor (CMOS) sensor, and is thus detected. Data is thus reproduced based on the detected image data.
A known method is disclosed in Nikkei Electronics Magazine, Jan. 17, 2005 Issue, pp. 106-114.
Hologram recording and reproducing method is known. In this method, the factors listed below greatly affect signal qualities (such as signal-to-noise ratio (SNR) and an error rate):
a) pixel defect on a spatial light modulator (SLM),
b) pixel defect on an image sensor,
c) image defect originating from an optical system,
d) media performance (variation in recording sensitivity, diffraction efficiency, etc.), and
e) optical system performance (variations in recorded image and reproduced image due to aberrations, signal quality degradation due to stray light leaking into a detection system).
In particular, factors a) through c) greatly affect signal quality during recording and reproducing operations. The SNR and the error rate are constantly degraded. The factor a) means a defect in recorded data, and the factor b) means a defect in reproduced data. The factor c) is generated by dust sticking to a lens or the like in an optical path. The image defects thus created are likely to affect adversely the signal quality.
In the known hologram recording and reproducing method, no techniques are available to manage the pixel defect and the image defect occurring during the recording and reproducing operations. There is a need for managing these defects to prevent persistent degradation in the SNR and the error rate.